This invention relates to an optical time domain reflectometer having a receiver with selectively controlled gain.
An optical time domain reflectometer (OTDR) is used for testing fiber optic cables. The OTDR comprises a laser diode which is used to introduce pulses of optical energy into an optical fiber at a proximal end of the fiber under test, and a photodiode which generates a current signal that depends on the power with which optical energy is emitted from the fiber at its proximal end in response to the input pulse. It is necessary to convert this current signal to a voltage signal in order to drive a cathode ray tube, which is typically used to provide a display in an OTDR. A transimpedance amplifier may be used to convert a current signal to a voltage signal. However, in a conventional OTDR the current signal is converted to a voltage signal by using the current signal to charge a capacitor that is connected between the input terminal of a voltage amplifier and ground. The resulting voltage signal is proportional to the time integral of the current signal. A series capacitor connected to the output terminal of the amplifier differentiates the voltage signal provided by the amplifier and generates a voltage signal of which the amplitude is proportional to the current signal generated by the photodiode.
Optical energy is emitted from the fiber at its proximal end due to reflections and Rayleigh back-scattering. Reflections occur due to abrupt changes in the refractive index of the medium through which the light pulse is propagating. Typically, such changes occur at connections between lengths of fiber and at breaks in the fiber. Back-scattering occurs due to interaction between the photons of the optical pulses introduced into the fiber and the molecules of the fiber. Back-scattering results in an unavoidable loss in power as an optical pulse travels along the fiber, and therefore the power level of back-scattered light establishes the maximum distance that a pulse can travel along the fiber without suffering an unacceptable loss in power. The power level of back-scattered light also provides diagnostic information, in that it is higher at locations where the fiber is under stress and might therefore be susceptible to damage.
The power level of back-scattered energy is very much lower than the power level of reflected energy. For example, if the duration of the pulse is such as to allow distance resolution by the OTDR of 1 m, the power level of back-scattered energy might be 50 dB below the power level of reflected energy.
When an OTDR is used to observe variation in the power level of back-scattered energy, it is desirable that the back-scattered energy utilize substantially the entire dynamic range of the OTDR. However, because the power level of the back-scattered energy is so low, it is necessary to amplify the signal detected by the photodiode before processing. If a reflection pulse is amplified to the same extent, the amplifier saturates, resulting in distortion of the signal even after the reflection pulse is no longer present. A previous attempt to overcome this problem has involved connecting a Schottky barrier diode between the cathode of the photodiode and ground. A current source that is connected to the Schottky barrier diode is triggered when a reflection pulse is received, and supplies current to the Schottky barrier diode, rendering the Schottky barrier diode conductive so that current provided by the photodiode is shunted to ground rather than being integrated on the capacitor. However, the current source introduces charge into the signal path and limits the accuracy of the measurements that can be made.
It is known to use a photoconductive switch in a high speed sampler. A known type of photoconductive switch comprises a die of InP having two interdigitated electrodes in ohmic contact with its top surface. The switch is turned on by illuminating the top surface of the die. Since holes and electrons are created in pairs, no net charge is generated when the switch is turned on. The ratio between the on resistance and off resistance of a photoconductive switch is typically 10.sup.-4.
U.S. Pat. No. 4,376,285 issued Mar. 8, 1983 (Leonberger et al) discloses a photoconductive switch having interdigitated electrodes. U.S. Pat. No. 4.490,709 issued Dec. 25, 1984 (Hammond et al) also discloses a photoconductive switch.